
312 STRUCTURAL AND FIELD GEOLOGY 
prominent feature, rising and falling in long, gentle undula- 
tions, that may not trend in any particular direction. In 
other cases, the surface is more diversified; and may show a 
pronounced “corduroy” or wrinkled configuration—being 
marked by a series of longer and shorter parallel and often 
interosculating banks with intervening hollows. The trend of 
these drums, or drumlins as they are called, should be 
carefully noted. In most cases the banks in question appear 
to be original, ze. they are forms assumed by the boulder- 
clay while it was being accumulated. Occasionally, however, 
they are simply the result of the unequal erosion of a gently 
undulating or plain-like surface of boulder-clay. 
The colour of the till and the nature of its included stones 
and boulders should be noted. The colour will usually be | 
found to correspond with that of the predominant rock or 
rocks of a district—it is therefore Jocal. The most abundant 
rock-fragments in the till are also generally local, but com- 
mingled with these many others of more distant derivation 
are sure to occur. The observer should take percentages of 
the different kinds of rock, and endeavour to ascertain their 
several sources. If he be a beginner he will naturally be at 
fault, but a good geological map of the country will afford him 
some help, and he may obtain more by examining the rock- 
collections in public museums. Should he be able to deter- 
mine the source of many of the stones which are “strangers,” 
this will give him a strong hint as to the general direction 
followed by the old mer de glace. 
Lenticular layers and sometimes thicker series of un- 
fossiliferous gravel, sand, and laminated clay, may occur 
underneath, and are still more frequently included in boulder- 
clay. Such deposits are often more or less confused and 
disturbed. They obviously point to the action of subglacial 
water. The boulder-clay that immediately underlies them 
will be found quite fresh and unaltered, showing that it has 
never been exposed to the oxidising influence of the atmos- 
phere. Now and again, however, stratified deposits of gravel, 
sand, loam, marl, peat, etc., are met with resting upon and 
covered by boulder-clay. The boulder-clay underneath such 
beds is almost invariably discoloured for some distance down- 
wards, thus showing that it must for some time have been 
